1. Field of the Invention
This invention relates generally to apparatuses and methods for recording information in an optical storage medium, and specifically to an apparatus and a method for recording information in an optical storage medium using a pulsed laser.
2. Description of Related Art
Optical storage media, such as DVD-R and Blu-ray (registered trademark) discs, in which information is retrievably recordable, typically adopts the dual-layer recording scheme in order to increase the storage capacity. In recent years, to enable recording in more than two layers, a recording method using a two-photon absorption compound has become a focus of study.
The two-photon absorption compound is a compound which absorbs light with electrons excited therein only when two photons simultaneously strike the compound. Therefore, the two-photon absorption compound induces reaction (absorption) with a probability proportional to the square of the intensity of the input light and with high resolution and selectivity in depth of focus. To be more specific, when a laser beam is focused into the optical storage medium having a recording layer containing a two-photon absorption compound, the two-photon absorption compound absorbs (reacts with) the input beam only in a small region around a focused point and never reacts in regions even slightly out of the small region in depth (in the direction of travel of the laser beam). In this way, the two-photon absorption compound is capable of inducing reaction in a small region in depth, and thus a recording layer made of this compound is suitable for recording information in multiple layers
It is desirable to use a pulsed laser such as a femtosecond laser instead of a hitherto prevalently used continuous-wave laser particularly when an optical storage medium containing a two-photon absorption compound is used to record information therein. This is because the pulsed laser can produce a laser beam having a peak power sufficient to induce two-photon absorption reaction even when it operates at a relatively low average power.
Since the conventional apparatus for recording information in an optical storage medium uses a continuous-wave laser, the timing control of the laser beam is not a matter of particular concern, and the operation timing of internal components of the apparatus are synchronized based upon a synchronizing signal which is generated for example by using a wobble signal (see JP 2005-122807 A).
However, in the apparatus where a pulsed laser is used, the timing of pulsing of a pulsed laser beam is difficult to control in some instances depending upon the type of the pulsed laser used. Therefore, disadvantageously, a synchronizing signal generated independently of the pulsed laser, as used in the conventional apparatus, could not be used to synchronize the internal components of the apparatus.
The present invention has been made against this backdrop.